In the prior art a technique known as Laser Doppler Velocimetry (“LDV”) uses the coherent nature of laser light to focus two crossing laser beams with identical polarisation at a single reference point, thereby creating linear and regularly spaced interference fringes within a defined measurement volume. An object passing through the measurement volume will reflect incident light from the fringes back to a detector via a lens system and produce a signal that can be interpreted to deliver the velocity of the object. The signal frequency will relate to the fringe spacing and the velocity of the object. For precision laser beam geometry, fringe spacing is highly regular, allowing accurate velocity measurements to be made. An instrument using this technique is commonly known as a Velocimeter.
For two laser beams emerging from the final focussing lens of a velocimeter at spacing L and each having a beam diameter δ, and where Π is the mathematical constant Pi (3.14159 . . . ), the number of fringes N generated in the measurement volume at the crossing point of the two beams is given by the formula:N=4/Π*L/δFor laser beams of wavelength λ and a focusing lens of focal length f, the diameter D of the measurement volume is given by the formula:D=4/Π*λf/δ
It will be appreciated by those skilled in the art that a focused laser beam forms a ‘beam waist’ at the point of focus of finite diameter D and length.
As an example, and to illustrate the later discussion, consider a focusing lens of focal length of 250 mm and a pair of laser beams with a beam spacing of 45 mm, beam diameter of 2 mm and a wavelength λ=780 nm. The number of fringes N and the measurement volume diameter D are thus:
N = 4/Π * L/δD = 4/Π * λf/δN = 1.273 * 45/2.0D = 1.273 * 780 * 10−9 * 250/2.0N = 28 fringesD = 124 microns
This gives a fringe spacing in the measurement volume of D/N=4.4 microns.
The orientation of the fringes in relation to a given polarisation axis of the laser beams is fixed, such orientation being a function of the interference effect which creates the fringes.
If the two laser beams do not cross precisely at their focus (known as the “beam waist”), the geometric regularity of the fringes will be compromised, and the fringe spacing will vary throughout the length of the measurement volume. An object travelling at a constant velocity will therefore create different frequencies as it passes through different parts of the measurement volume, an undesirable effect. In the above example, the measurement volume length will be less than 1 mm, and the manufacturing precision of the lenses and optical components used in the velocimeter must of necessity be of a very high standard (and therefore expensive) to achieve consistency of fringe spacing.
Using the above example, an object passing through the measurement volume will produce a signal with a velocity constant Fout determined byFout=N/D Fout=0.227*106 Hz/meters/second
As velocimetry can be used to measure the velocity of high speed particles with very small sizes (sometimes having sub-micron diameters), the sensitivity of the detectors used must be high, as the amount of light scattered by the particle as it passes through a fringe will be small. In order to measure high velocities, the bandwidth of the detectors used must also be high. The gain-bandwidth product of the detector will therefore be high, increasing the cost. In a limiting case, the cost of the detector required might be so high as to render the use of velocimetry uneconomic for some applications.
Again in the above example, a detector with sufficient sensitivity to measure (say) high speed smoke and debris efflux in the 1-3 micron diameter range from a small rocket motor might only have a bandwidth of 10 MHz. The maximum velocity capable of being measured would therefore be 10/0.227=44 meters/second. However, combustion efflux velocities can easily reach several hundred meters/second, and for very powerful rockets, velocities of several thousand meters/second can be encountered. The bandwidth limitation would not allow the instrument to be used.
As the direction of the passing object relative to the velocimeter might vary, or in some circumstances might not be known, it may be necessary to rotate the entire velocimeter to ensure that the orientation of the fringes is at right angles to the object path. This may not always be possible, or in turn might cause the laser beams to collide with other (unspecified) objects within the field of view of the instrument, causing unwanted backscatter of laser light into the instrument.